The present invention relates to processing equipment for the treatment of sewage wastewater or other contaminated water with microorganisms to remove impurities and in particular, to processes of this type utilizing jet aeration for wastewater reaction and mixing and including an oxygen permeable membrane in an air line leading to the jet aeration apparatus so as to supply a relatively highly concentrated oxygen gas stream to the reactor.
Oxygen is an essential component in any activated sludge or fixed film process for the treatment of wastewater. Microorganisms in such processes require oxygen to sustain their metabolic function allowing them to multiply and so as to consume and digest contamination in the water as food.
Most wastewater treatment facilities treat large amounts of wastewater each day, therefore, large quantities of oxygen are also required. For example, in a sequencing batch reactor process having two reactor basins that is designed to treat one million gallons of water each day, the oxygen requirement may be on the order of 4,000 pounds per day. Typically, oxygen is supplied to the wastewater treatment process by bubbling air through the wastewater so that the oxygen interacts with the sludge or film. When air is used to supply the oxygen, the total gas amount pumped into the reactors may exceed 20,000 pounds per day as some oxygen is lost and because oxygen is only a partial component of air. In particular, air contains approximately twenty-three percent (23%) oxygen by weight with most of the remainder being nitrogen. As some of the oxygen and relatively little of the nitrogen do not completely dissolve in the water in conventional processes, relatively large quantities of undissolved gas escape from the top of the wastewater.
Nitrogen is an inert gas, only slightly soluble in water and has, therefore, been considered relatively unimportant in the treatment of wastewater. However, when air is used as the oxidant in a treatment process, the large quantity of nitrogen vented into the atmosphere results in substantial heat loss from the reactor. Heat loss from the processing wastewater due to warming of the nitrogen/oxygen off-gas and to wastewater evaporation into the off-gas is undesirable because aerobic digestion normally progress more efficiently at warmer temperatures.
A major disadvantage of using air as the oxidant in an activated sludge treatment process, fixed film process, or the like is that wastewater suspended in the nitrogen/oxygen off-gas forms aerosols that, along with any voltatiles escaping with the off-gas, may carry toxic or hazardous substances from the wastewater that either pollute the surrounding air or must be scrubbed, filtered or otherwise removed from the off-gas stream before venting. The added volume of the off-gas, due especially to the nitrogen present when air is used, at best substantially increases the size of piping and other processing equipment and operational costs as compared to what would be required if air were not used as a source for the oxygen.
When air is replaced by an oxygen rich gas in the wastewater treatment process, heat loss can be greatly diminished resulting in a more efficient treatment process that can be maintained at a relatively higher temperature without the addition of external heat. The total volume of gas bubbling through the wastewater is also lessened, resulting in a smaller volume of hazardous gas emission requiring smaller off-gas piping and equipment, smaller gas filtration equipment and reduced scrubbing costs.
In a sequencing batch reactor (SBR) process, replacement of air by oxygen rich gas is particularly beneficial. An SBR usually includes two or more batch reactor basins containing activated sludge. A specific volume of wastewater enters a basin during a "fill" cycle. Upon completion of a fill cycle, a "react" cycle normally begins wherein the wastewater is mixed with the activated sludge and aerated. Following the react cycle, the liquid in the basin is allowed to settle and then the treated wastewater is decanted.
Ideally, the fill cycle is anoxic, or without aeration so that the oxygen demand in the basin is high, producing a large oxygen driving gradient when aeration begins during the react cycle, resulting in a more efficient process.
Although the conventional use of air to provide oxygen has created a number of problems that are now increasing with air pollution concerns, air pumped directly into the reactor has remained the normal source of oxygen in wastewater treatment because of problems associated with producing oxygen rich gas and/or transporting the gas to the wastewater.
Prior art attempts have been made to eliminate the use of air while providing a generally "purified" supply of oxygen. Such prior art attempts have included use of bottled or cryogenic oxygen, molecular sieves and the placement of an oxygen separation membrane directly in the wastewater. However, bottled and cryogenic oxygen are relatively expensive substantially adding to the operating costs of such processes. The other prior art methods have been relatively inefficient. This has been especially true of the previous use of an oxygen permeable membrane that was placed directly in the wastewater reactor.
In particular, when the oxygen permeable membrane is placed at the interface between the air and the wastewater in a reactor basin, it is difficult to maintain the suitable operation membrane in a reactor filled with wastewater. That is, the membrane quickly becomes covered with slime and becomes clogged. Therefore, a reactor using such a membrane must include access to equipment to allow for frequent cleaning, resulting in increased reactor, equipment and maintenence costs and substantial downtime.
The prior art has most likely attempted to resolve the oxygen concentration problem by placement of the membrane directly in the wastewater to avoid the explosive hazard that potentially exists in the pumping or compressing of relatively highly concentrated oxygen. Consequently, pumps or compressors have been maintained upstream of the membrane and the membrane has been placed directly in the wastewater.